1. Field of the Invention
This invention relates to a rotary cordierite heat regenerator and a method of producing the same. More particularly, the invention relates to a rotary cordierite heat regenerator based on a honeycomb structural body which has been used as industrial heat exchangers or as a part of internal combustion engines or external combustion engines such as gas turbine engines and Stirling's air engines.
2. Description of the Prior Art
In general, a rotary ceramic heat regenerator comprises a cylindrical matrix of honeycomb structure with a diameter of 30-200 cm and a matrix-holder ring to be fitted on the outer circumference of the cylindrical matrix, and the heat regenerator is rotated in a two-passage chamber, which chamber is divided into two sections by a dividing means, i.e. a section defining a heating fluid passage and another section defining a recovering fluid passage. The heat regenerator rotating has a chamber divided into two section defining a heating fluid passage and another section defining a recovering fluid passage, and it cyclically repeats the storing and the releasing heat in the chamber for facilitating heat exchange.
Thus, for manifesting characteristics of the rotary ceramic heat regenerator, it is required to have a high heat exchange efficiency and a low pressure loss so as to ensure smooth passage of heating and recovering fluids therethrough.
A typical ceramic rotary heat regenerator of the prior art is disclosed by the U.S. Pat. No. 4,304,585. This U.S. Patent teaches a method of producing a rotary ceramic heat regenerator by firing a plurality of matrix segments of honeycomb structural body, bonding the thus fired matrix segments to form a rotary heat regenerator by a ceramic bonding material having substantially the same mineral composition as that of the matrix segments after firing, the ceramic bonding material having a thermal expansion that is less different from that of the matrix segments after firing, and firing the thus bonded matrix segments. Of the rotary ceramic heat regenerators thus produced by the method of this U.S. Patent, a rotary cordierite heat regenerator has a particularly high thermal shock resistance because it has a small coefficient of thermal expansion. Besides, the rotary cordierite heat regenerator thus produced has a high chemical inertness which has been experienced in those lithium aluminosilicates, such as .beta.-spodumene, which have a similar low thermal expansion to that of cordierite.
Generally speaking, it is difficult to sinter cordierite to a dense structure. Especially, in case of low-expansion cordierite body with a coefficient of thermal expansion smaller than 2.0.times.10.sup.-6 /.degree.C. over a range of room temperature to 800.degree. C., the content of fluxing ingredients such as calcia, alkali, potash, soda, and the like must be limited to a very low level, so that vitreous phase therein is very scarce and the cordierite tends to become porous. More particularly, cordierite honeycomb structural bodies which have been used in recent years as catalyst-carriers for purifying automobile exhaust gas are required to have a coefficient of thermal expansion smaller than 1.5.times.10.sup.-6 /.degree.C. over a range of room temperature to 800.degree. C., so that the porosity of the sintered cordierite body is 20-45% at the least even if the starting materials, such as talc, kaolin, alumina or the like including the place of their production, their chemical composition, their particle size, and the like, are carefully selected to have only a small amount of impure ingredients. Accordingly, a rotary cordierite heat regenerator made of the above-mentioned cordierite matrix of honeycomb structural body has a serious problem of low heat exchange efficiency because fluid leakage is likely to occur between the heating fluid passage and the recovering fluid passage leading therebetween or through open pores of the partition walls defining the channels of the honeycomb structural body. The low heat exchange efficiency of the rotary heat regenerator tends to deteriorate the overall heat exchange efficiency of a large system having such a rotary heat regenerator.
On the other hand, if the porosity of cordierite is reduced, the thermal expansion thereof tends to increase. For instance, British Patent Specification GB-2071639A proposes a method of reducing the porosity by applying a glaze or the like on the surface of partition walls defining channels of the porous honeycomb structural body. This method has a shortcoming in that the flux components contained therein tend to cause a large increase of the thermal expansion and deteriorate the thermal shock resistance. Conventional methods of producing cordierite matrix segments of honeycomb structural body with a comparatively low porosity have a shortcoming in that a large shrinkage is caused in the drying and firing stages, and such shrinkage tends to form cracks in the segments. Accordingly, it has been difficult to produce large matrix segments with a reasonably high yield.